Mild hypothermia depresses conductivity and pacemaker activity. As the body temperature falls, there is a progressive prolongation of the PR, QRS, and QT intervals, and non-specific changes in the ST segment and T waves. Broadening of the QRS complex may be due to the appearance of J waves. The Osborn J wave or camel-hump sign is typical of hypothermia, and is found consistently when the temperature is below 25 °C ( Fig 1). It is a secondary wave following the S wave. This extra deflection between the QRS complex and the ST segment is upright and more prominent in AVL and left chest leads. As the body temperature falls, its amplitude increases and T waves become inverted in the same leads. There are several hypotheses for the origin of the J wave: injury current, delayed ventricular depolarization, or early repolarization before delayed depolarization has occurred. It is so characteristic that its presence should prompt the search for hypothermia; differential diagnosis includes early repolarization, cerebral injury, and vasospastic angina.
Fig. 1 ECG of an 87-year-old woman found with rectal temperature of 24.5 °C. The heart rate is irregular between less than 32 and 38 beats/min with no P wave. There is marked prolongation of the QT interval and oscillation of the baseline, possibly due to muscle tremors. The typical Osborn J waves can be seen in almost all leads.
Atrial fibrillation secondary to atrial distension, atrial flutter, atrioventricular junctional rhythm, and ventricular premature beats commonly appear below 30 °C. Ventricular fibrillation can occur below 28 °C, and asystole may appear at and below 20 °C.
Oscillation on the ECG baseline is due to muscle tremors and may be seen even in the absence of clinically obvious shivering. Central nervous system and neuromuscular abnormalities
The metabolic rate of the brain declines in parallel with the fall in temperature, improving the chances of survival even after prolonged periods of circulatory arrest. Blood flow declines secondary to a fall in cardiac output, increased vascular resistance, and blood viscosity. Enzymatic and biological reactions slow down and cerebral function is depressed. Although the clinical expression may be variable, exhaustion, with tremulous speech and ataxia, may be noticed at 32 °C. Dysarthria, stupor, hallucinations, and dysphoria may appear with moderate hypothermia. Paradoxical undressing and aggressive behavior have been reported. At 30 °C, the patient is able to answer simple questions, but responds very slowly. Coma, hypotony, absence of deep tendon reflexes, and fixed and dilated pupils are present below 28 °C. Electrical activity progressively decreases below 28 °C and ceases completely around 15 to 20 °C with a silent electroencephalogram.
Muscle shivering reaches its peak at 32 to 33 °C and disappears completely at 30 °C. In severe hypothermia, muscle hypotony followed by rigidity occurs.
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